1. Field of the Invention
Gas turbines require the highest standards of durability. The critical component is the blade in which layers for protection against erosion, wear, corrosion and oxidation at high temperatures are acquiring importance. The protective layer usually has a shorter service life than the core material of the blade, for which reason the renewability of the former is coming more and more to the fore.
The invention relates to the further development of processes for repairing, maintaining and renewing heat engine components rendered unusable by erosion, wear, corrosion, oxidation or mechanical damage and provided with protective layers. Under these circumstances, the old, existing protective layer first has to be removed, and this can in principle be carried out mechanically or chemically. The chemical method quite generally occupies a leading position in the field of surface modification by stripping.
In particular, it relates to a process for chemically stripping a surface-protection layer with a high chromium content from the main body of a component composed of a nickel-based of cobalt-based superalloy.
2. Discussion of Background
The removal of protective layers on main bodies (substrate) composed of superalloys is carried out in a conventional manner, inter alia, by the electroless chemical dissolution process by the action of solutions which contain oxidizing acids as an important component. Thus, the use of HNO.sub.3 -containing solutions for dissolving protective layers containing nickel aluminides is generally recommended (cf. US-A-4,425,185; AU-B-10761/76; US-A-4,339,282; US-A-3,607,398; US-A-3,622,391; US-A-3,833,414). Other oxidizing solutions contain, for example H.sub.2 O.sub.2 and are used for stripping nickel (cf. US-A-4,554,049). The use of solutions which contain nitrobenzinesulfonic acid and Na compounds for chemically stripping so-called "aluminum diffusion layers" on blade materials is furthermore known (cf. EP-A-0,161,387). Furthermore, solutions containing iron sulfate and hydrochloric acid are recommended for removing chromium- and aluminum-containing protective coatings on a cobalt base, the iron sulfate having an oxidizing action either directly or by way of hydrolysis as sulfuric acid (cf. DE-B-2,717,435). In addition, solutions containing HNO.sub.3 and HF have already been used for stripping chromium- and aluminum-containing or aluminum-containing protective layers from nickel or cobalt based alloys having a chromium content exceeding 18% (cf. US-A-3,458,353).
The known processes employing oxidizing solutions are based on the emperical fact that they only attack the core material of the main body, in the present case a nickel-based or cobalt-based superalloy, weakly if it contains at least 7% by weight of Cr. A process in which the main body is also stripped in addition to the protective layer is, of course, in most cases unusable in practice.
As a result of the changeover to increasingly higher Cr contents in the protective layers, however, the relationships of the electrochemical potentials of the core material to that of the protective layer is precisely reversed: The protective layer becomes positive compared with the main body in an oxidizing solution. This has the result that the protective layer cannot be removed either electrolytically or by electroless chemical means. The main body is preferably always attacked, whereas the protective layer to be removed resists longer. The above-mentioned known processes cannot therefore be used on the modern material combinations comprising a protective layer with high chromium content/a superalloy with moderate chromium content.